(a) Field of the Invention
The present invention relates to photoacoustic spectroscopy, and more particularly, to applications that involve measurement of two or more gases or vapors in a mixture, and real-time gas monitoring.
(b) Description of the Related Art
Photoacoustic measurement is based on the tendency of molecules, when exposed to certain frequencies of radiant energy (e.g. infrared radiant energy), to absorb the energy and reach higher levels of molecular vibration and rotation, thereby reaching a higher temperature and pressure. When the radiant energy is amplitude modulated, the resulting dissipated heat fluctuations from energy absorption produce corresponding temperature and pressure fluctuations. A sensitive microphone can be used to generate an electrical output representing pressure fluctuations. The amplitudes of the acoustic signal and resulting electrical output are proportional to the intensity of the radiation and the concentration value of the absorbing gas.
A variety of these devices are known, for example U.S. Pat. No. 4,557,603 (Oehler et al), U.S. Pat. No. 4,818,882 (Nexo et al), U.S. Pat. No. 5,933,245 (Wood et al), U.S. Pat. No. 6,006,585 (Forster), and U.S. Pat. No. 6,148,658 (Chou et al). The devices have several components in common. In particular, laser or other energy sources produce radiant energy which is modulated either thermally (power on/off) or with a chopping device. The modulated energy is provided to a cell containing a gas or gas mixture that absorbs the radiant energy, leading to temperature fluctuations in the gas that track the modulation frequency. Temperature is not sensed directly, but rather pressure fluctuations that accompany the temperature fluctuations are detected by a sensitive microphone in the cell. The microphone output is detected at the modulation frequency to provide an electrical signal proportional to the gas concentration.
There is a need to determine the concentrations of one or more gases within a gas mixture. While this could be accomplished with two or more sensing systems, one devoted to each of the gases under study, a sharing of components among several systems would likely reduce costs. Accordingly there have been several proposals involving use of a single photoacoustic cell to detect two or more gases.
For example, the Nexo et al patent discloses a perforated disk with three sets of filter openings with different spacing between adjacent openings, for simultaneously filtering infrared light into different wavelengths “absorbed by N2O, SO2, and anesthetics, respectively” and modulating the wavelengths at three different frequencies. Signals corresponding to the various gases are said to be separated through electric filtration of the microphone signal.
The Oehler et al patent discloses a mechanical light modulator and monochromator having different interference filters said to enable simultaneous and separate detection of several components of a gas mixture. Oehler indicates that the interference with measurement by other gas components can be largely eliminated by using more than one narrow-band filter adapted to the maxima or flanks of the measuring gas or interfering components. Concentrations of different components are said to be determinable from the measurements performed with the different narrow band filters, with these filters being successively introduced into the path of the rays.
One disadvantage of these systems is the need to provide the radiant energy in extremely narrow bands. This requires either lasers for generating energy, or equipment designed to successively introduce different narrow-band filters into the light path between the source and photoacoustic cell. Either approach adds to the cost of the system. Further, it is difficult within the confines of these systems to distinguish between two gases with overlapping or coinciding absorption bands, or to determine the presence of an unknown absorbing gas.
Therefore, it is an object of this invention to provide the capability of separately measuring the concentration values of several gases having absorption lines or bands which may overlap or coincide with one another, or to detect the presence of another gas whose absorption bands or lines may overlap or coincide with those of the several gases of which sensing is desired.
The systems of the present invention do not suffer from the problems and drawbacks of other analysis techniques such as mass spectroscopy. Many current chemical analysis techniques are often unsatisfactory in that they are slow and expensive.
On-road vehicle emission inspections for pollutants are also important in order to intercept major pollution offenders and to improve overall air quality. Again, a fast, reliable, and accurate method of detection and a detector would be valuable tools to combat this source of air pollution.
Other examples of where a detector and method which is fast, economical, and reliable would be important are in sampling air proximate to a natural gas pipeline to determine the presence of leaks. Soil samples which may contain certain dense non-aqueous phase layer chemicals (DNALP) such as chlorobenzene and other pollutants are also in need of a detection method, system, and detector which is fast, economical, and reliable to detect the presence of such pollutants.
As another example, in the military or security fields, a fast; reliable, and accurate detector and method is needed to determine the presence of explosives or chemical warfare compounds. The foregoing are but several examples where there is a need for a reliable, inexpensive detector which does not suffer from the drawbacks of mass spectrometry or gas chromatography.